Because of the well-known and clearly established activity of 1.alpha.-hydroxyvitamin D compounds in regulating calcium and phosphate homeostasis in the animal or human, there has been interest in the preparation of the natural metabolites and in the discovery of analogs with useful medicinal properties. This has led to the preparation of a variety of compounds (for examples, see DeLuca et al., Topics in Current Chemistry, vol. 83, p. 1 (1979); Ann. Rev. Biochem. 52, 411 (1983); Yakhimovich, Russian Chem. Rev. 49, 371 (1980) some of which, e.g. 1.alpha.-hydroxyvitamin D.sub.3 (U.S. Pat. No. 3,741,996) or 1.alpha.,25-dihydroxyvitamin D.sub.3 (U.S. Pat. No. 3,697,559) already find use in medical practice. Interest in such compounds is continuing especially now that it has been recognized that in addition to their classical function as regulators of calcium homeostasis, 1.alpha.,25-dihydroxyvitamin D.sub.3 and its analog, 1.alpha.-hydroxyvitamin D.sub.3, also affect cellular differentiation processes and are capable of inhibiting the growth and proliferation of certain malignant cells [ Suda et al., U.S. Pat. No. 4,391,802; Suda et al., Proc. Natl. Acad. Sci. USA 80, 201 (1983); Reitsma et al., Nature, Vol. 306, p. 492-494 (1983)]. There is increasing evidence to show, that expression of biological activity by vitamin D metabolites and analogs involves binding to an intracellular receptor protein at some stage of the overall process (see DeLuca et al., supra). High affinity for this receptor protein is thus a prerequisite for high potency, and desirable vitamin D analogs are those which compete effectively with the natural hormone, 1,25-(OH).sub.2 D.sub.3, for the receptor binding site.
Known side chain unsaturated vitamin D compounds include the hydroxy derivatives of vitamin D.sub.2, namely 25-hydroxyvitamin D.sub.2 (U.S. Pat. No. 3,585,221), 1.alpha.,25-dihydroxyvitamin D.sub.2 (U.S. Pat. No. 3,880,894), 24-hydroxy- and 24,25-dihydroxyvitamin D.sub.2 (Jones et al., Arch. Biochem. Biophys. 202, 450 (1980)), 1.alpha.-hydroxyvitamin D.sub.2 (U.S. Pat. No. 3,907,843) and certain 24-demethylvitamin D.sub.2 compounds (U.S. Pat. No. 3,786,062; Bogoslovskii et al., J. Gen. Chem. USSR 48(4), 828 (1978); Chem Abstr. 89, 163848j and 89, 209016s). One example of a compound with a cis-double bond in the side chain is also known (Bogoslovskii et al., supra).